Various optical systems forming photomagnetic signal detecting apparatus have been already suggested. For example, in the specification of a Japanese patent laid open No. 191156/1984, in an optical system wherein a reflected polarized light from a magnetic optical recording medium is led to a beam splitter and the transmitted light or reflected light by this beam splitter is divided into two components intersecting at right angles with each other by a polarizing beam splitter and is then received by a detector, a rotator is provided between the beam splitter and the polarizing beam splitter to easily adjust the polarization plane of the polarized light entering the polarizing beam splitter.
Also, in the specification of a Japanese patent laid open No. 113347/1985 a beam splitter is provided in a light path and is multilayer-coated on the reflecting surface with a dielectric film with optical characteristics having a relationship relation of T.sub.P &gt;T.sub.S between the amplitude transmissivity T.sub.P for a P polarized light and the amplitude transmissivity T.sub.S for an S polarized light and a phase difference of n.pi.-.pi./4.ltoreq..delta..ltoreq..pi.+.pi./4 (wherein n is an integer) between the transmitted P wave and S wave to increase the magnetic optical rotation angle and to make reproduction easy.
Further, in the specification of a Japanese patent laid open No. 182537/1985, there is disclosed a technique whereby a polarizing beam splitter is provided rotatably with respect to the optical axis and a differential amplifier, detecting and differentially amplifying the two separate lights emitted from this polarizing beam splitter, is provided so that the variation of the polarization plane of the polarizing beam splitter may be detected.
A conventional photomagnetic signal detecting apparatus shall be explained with reference to FIGS. 14 to 19. FIGS. 14, 18 (a) and (b) and 19 are explanatory views showing schematic fundamental formations. FIGS. 15, 16 and 17 are views explaining light vectors.
In FIG. 14, for example, a P-polarized light beam having come out of a laser light source 1 is made into a parallel light beam by a collimator lens 2, passes through a beam splitter 3 and is radiated by an objective lens 4 onto a recording medium surface 5. Depending a whether the magnetizing direction of the recording medium surface 5 is upward or downward, the reflected light by the recording medium surface 5 is subjected to a Kerr rotation .theta.k or -.theta.k to be a light vector P.sub.1 or P.sub.2 as in FIG. 15.
The reflected light by the recording medium surface 5 passes through the objective 4, is reflected by the beam splitter 3 and enters a 1/2-wavelength plate 6.
In the 1/2-wavelength plate, in order that the light amounts of the divided transmitted light and the reflected light by a later described polarizing beam splitter (abbreviated as the PBS hereinafter) 16 may be substantially equal to each other, the polarization plane of the light entering the PBS 16 is rotated by 45 degrees with respect to the polarization plane before the incident light is subjected to the Kerr rotation as in FIG. 16. In order to rotate the polarization plane by 45 degrees, the orientation angle .PHI. made by the main axis of the 1/2-wavelength plate 6 and the polarization plane of the incident light is set at 22.5 degrees. If the adjusting error of this orientation angle .PHI. is .alpha., the actual orientation angle will be set at .PHI. 22.5 .degree..+-..alpha., the polarized light plane will rotate by 45.degree..+-.2.alpha. and the error will be twice as large. Therefore, the operation of setting the orientation angle at 22.5 degrees requires high precision.
The light beam having passed through the 1/2-wavelength plate 6 and having had the polarization plane rotated by 45 degrees enters the PBS 16. As the PBS 16 transmits a P polarized light and reflects an S polarized light, as in FIG. 17, the transmitted light will be only P components (P.sub.1P and P.sub.2P) and the reflected light will only be S components (P.sub.1S and P.sub.2S). The components P.sub.1P, P.sub.2P and P.sub.1S, P.sub.2S divided by the PBS 16 are received respectively by photodetectors 17a and 17b and are converted to electric signals. The intensity of this electric signal will be high at the time of P.sub.1P but will be low at the time of P.sub.1S on the photodetector 17a and will be low at the time of P.sub.1S but will be high at the time of P.sub.2S on the photodetector 17b. Therefore, if the difference between the photodetectors 17a and 17b is taken by a differential amplifier 9, an information signal discriminating P.sub.1 and P.sub.2 from each other will be able to be extracted.
FIGS. 18 (a) and 18 (b) are explanatory views when the PBS 16 is rotated by 45 degrees, without using the 1/2-wavelength plate 6, to have the same effect as having passed through the 1/2-wavelength plate 6. FIG. 18 (b) is a sectioned view as seen in the direction of the arrow B in FIG. 18 (a). In this case, the plane formed by the two divided lights emitted from the PBS 16 and the paper surface of FIGS. 18 (a) form an angle of 45 degrees and photodetectors 17a and 17b also must be arranged in the plane formed by the two divided lights. Now, generally, in view of the optical axis adjustment, metal working and assembly, it is not easy and is preferable to be avoided to arrange an optical member at other intermediate odd angles than 0, 90, 180, 270 and 360 degrees.
FIG. 19 is an explanatory view when a Wollaston polarizing prism 18 is used instead of the 1/2-wavelength plate 6 and PBS 16. Instead of rotating the polarization plane by 45 degrees with the 1/2-wavelength plate 6, the Wollaston polarizing prism 18 is rotated by 45 degrees so that the normal light and abnormal light having passed through the Wollaston polarizing prism may be substantially of the same light amount and the separated lights are received by a two-divided photodiode 8. In this system, even though the Wollaston polarizing prism 18 is rotated by 45 degrees, as the separating angle of the divided polarized lights is small, the form of the optical system will not be complicated and, as the two-divided photodiode 8 is used, the number of parts is low. However, there are defects that the Wollaston polarizing prism 18 is much more expensive than the PBS 16 and the separating angle of the two polarized lights is so small that, in order to perfectly separate the polarized light, the light path must be taken to be large and the optical system becomes large.